Catalysts for olefin polymerization can be prepared by many processes, by reacting a solid component, which, as a rule, contains magnesium, titanium and a halogen, preferably chlorine, with an organoaluminium compound. The activity and the stereospecificity of such supported catalysts are usually improved by incorporating an electron donor (Lewis base) in the carrier component and by complexing the organoaluminium compound with an additional electron donor. It is known that magnesium alkoxides can be used as starting materials for the preparation of such supported catalysts. In this case, the magnesium alkoxide particles are preferably halogenated with the aid of a suitable halogenating agent, such as benzoyl chloride, thionyl chloride or titanium tetrachloride, in the presence of an electron donor, such as, for example, ethyl benzoate. If halogenating agents other than titanium tetrachloride are initially used, the latter must be incorporated in the solid component in the required amount of subsequent reaction with titanium tetrachloride, a content of 2 to 4.5% by weight of titanium usually being required. Regarding the details of the catalyst preparation, reference may be made to European Offenlegungsschrift 0,216,402 and 0,236,082 and to the literature cited there.
Since, in the polymerization of .alpha.-olefins, the morphology of the resulting polymer particles is a faithful reproduction of the morphology of the catalyst and this in turn is an exact image of that of the magnesium alkoxide used (see for example, J. Rudolph and J. Gross, Die Angewandte Makromolekulare Chemie 36 (1974), 195-197), it is also necessary to use a magnesium alkoxide of defined morphology to achieve defined properties of the desired polymer.
European Offenlegungsschrift 0,236,082 describes the preparation of spherical to raisin-shaped particles of magnesium alkoxides by conventional spray-drying. A preferably alcoholic solution of a carboxylated magnesium alkoxide is sprayed through a nozzle, which is not defined in detail, or via a rotating disk into a hot accompanying gas; this accompanying gas can be fed cocurrently or countercurrently. Although temperatures of 40.degree. to 120.degree. C. may be used, the range from 50.degree. to 90.degree. C. is preferred for ethanolic solutions since it is only in this range that the desired spherical to raisin-shaped particles can be produced. As is evident from the Examples, on the other hand, hollow spheres are obtained at 100.degree.-120.degree. C., many of which disintegrate into nutshell-like fragments.
Virtually all of the particles obtained by the process of European Offenlegungsschrift 0,236,082 have a diameter in the range of from 2 to 250 .mu.m; preferably, 90% by volume of the particles are in the range from 10 to 40 .mu.m, with a volume average at about 20 .mu.m.
After spray drying, the bound CO.sub.2 is expelled again by heating for several days in a stream of nitrogen at temperatures of, initially, 70.degree. C. to, finally, 150.degree. C. As an alternative to this expensive process, the carboxylated magnesium alkoxide obtained is said to be capable of being directed reacted further. The catalyst obtained is said to be capable of being used both for gas-phase polymerization and for liquid-phase polymerization. The process described in European Offenlegungsschrift 0,236,082 for the preparation of spherical particles of magnesium alkoxide is unsatisfactory for several reasons:
The preparation of more finely divided magnesium alkoxide having a mean diameter of less than 10 .mu.m, as is desirable especially for the preparation of catalysts for the liquid-phase polymerization, is not possible by this method. PA1 The relatively low drying temperature during spray drying leads to products which have a considerable residual moisture content, with the danger of agglomeration and caking on the walls. PA1 An inconvenient aftertreatment is required to remove the bound CO.sub.2. Alternatively, it is possible to dispense with this aftertreatment, but this gives a catalyst having substantially lower activity. PA1 1) An ethanolic suspension of magnesium ethoxide is prepared from magnesium and ethanol; the product is isolated and dried. PA1 a) is sprayed via a two-material nozzle with internal mixing of the type described in, for example, German Patent 2,627,880, and which is operated in the part-load range at 5 to 30%, preferably 10 to 25%, of capacity, PA1 b) into an inert accompanying gas which is under a pressure of 1.0 to 1.2 bar, preferably 1.01 to 1.05 bar, is fed cocurrently and has been preheated to a relatively high temperature of 100.degree. to 140.degree. C., preferably 105.degree. to 120.degree. C., it being advantageous if the spray drier is in the form of a long tube tapering conically downwards, PA1 c) after which the resulting finely divided carboxylated magnesium alkoxide is dried and is decarboxylated.
European Offenlegungsschrift 0,216,402 describes a multi-stage process for the preparation of spherical, mixed magnesium alkoxide of the formula Mg(OR).sub.2-a (OR').sub.a (a=0 to 0.5). In the preferred case (R=C.sub.2 H.sub.5 ; R'=CH.sub.3), the following procedure is adopted:
2) The dried magnesium ethoxide is dissolved in methanol; the solution is spray-dried at 15.degree. to 200.degree. C., preferably at 30.degree. to 70.degree. C., spherical particles having a diameter of from 5 to 30 .mu.m, preferably 10 to 18 .mu.m, being formed. At this point, a mixed alkoxide predominantly containing methoxide groups is present; it is unsuitable in this form for the preparation of olefin polymerization catalysts.
3) The product from 2) is suspended in ethanol, and the methanol formed at the equilibrium is distilled off.
4) The product having a low content of methoxide groups is isolated and dried.
Apart from the many process stages, such as, for example, changing the solvent twice and drying three times, the low solubility of the magnesium ethoxide in methanol (see European Offenlegungsschrift 0,216,402, Example 1) and the resulting necessity to evaporate large amounts of solvent with corresponding consumption of energy, are found to be a serious disadvantage. Furthermore, the product still has a residual content of undesirable methoxide groups.